Thermal detectors operate by absorbing energy from incident electromagnetic radiation and by converting the absorption-generated heat into an electrical signal indicative of the amount of absorbed radiation. Perhaps the most prominent type of thermal detectors currently available is uncooled microbolometer detectors, or simply microbolometers. A microbolometer is typically based on a suspended platform or bridge structure having a low thermal mass, which is held above and thermally insulated from a substrate by a support structure. The platform is provided with a thermistor, which is a resistive element whose electrical resistance changes in response to temperature variations caused by the absorbed radiation. The thermistor may, for example, be composed of a material having a high temperature coefficient of resistance (TCR), such as vanadium oxide and amorphous silicon. Because they do not require cryogenic cooling, uncooled microbolometers can operate at room temperature, which makes them well suited for integration within compact and robust devices that are often both less expensive and more reliable than those based on cooled detectors.
Arrays of uncooled microbolometers can be fabricated on a substrate using common integrated-circuit fabrication techniques. Such arrays are often referred to as “focal plane arrays” (FPAs), while the individual microbolometers forming the arrays may be referred to as “microbolometer pixels”, or simply “pixels”. In most current applications, arrays of uncooled microbolometer pixels are used to sense radiation in the infrared region of the electromagnetic spectrum, usually in the mid-wave infrared, encompassing wavelengths of between about 3 and 5 micrometers (μm), or in the long-wave infrared, encompassing wavelengths of between about 8 and 14 μm. These arrays are often integrated in uncooled thermal cameras for sensing incoming infrared radiation from a target scene. Each microbolometer pixel absorbs some infrared radiation resulting in a corresponding change in the pixel temperature, which in turn produces a corresponding change in electrical resistance. A two-dimensional pixelated thermal image representative of the infrared radiation emitted from the scene can be generated by converting the changes in electrical resistance of each pixel into an electrical signal that can be displayed on a screen or stored for later viewing or processing. By way of example, state-of-the-art arrays of infrared uncooled microbolometer arrays now include 1024 by 768 pixel arrays with a 17-μm pixel pitch.
In the last decade, there has been a growing interest in extending uncooled microbolometer spectroscopy and sensing applications beyond the traditional infrared range, namely in the far-infrared and terahertz (or sub-millimeter) spectral regions. As known in the art, these regions of the electromagnetic spectrum have long been relatively unused for industrial and technological purposes at least partly due to the lack of efficient techniques for detection and generation of radiation in this spectral range.
Extending the absorption spectrum of uncooled microbolometers beyond 30-μm wavelength is not straightforward, notably because the materials used to fabricate the detectors absorb predominantly in the infrared, and also because the pitch of terahertz-sensitive pixels is typically larger than that of infrared-sensitive pixels to avoid diffraction effects. Additionally, in order to optimize radiation absorption in the desired spectral band, conventional infrared microbolometer detectors generally include a reflector deposited on the underlying substrate to form a quarter-wavelength optical resonant cavity with the suspended platform. However, forming such a resonant cavity for detecting electromagnetic radiation at wavelengths longer than 10 μm is generally not practical with surface micromachining techniques commonly used in the fabrication of uncooled microbolometers.
Several approaches have been devised in order to improve the spectral response of uncooled microbolometers beyond 30 μm. One approach that has been studied and used in different applications relies on broadband thin-film absorbers, such as metallic blacks, organic blacks, and carbon nanotubes. However, fabricating these thin-film absorbers requires special deposition and processing techniques, which are generally not fully compatible with standard microfabrication and packaging processes of uncooled microbolometers.
Another approach is based on antenna-coupled microbolometer detectors, in which the electromagnetic radiation is absorbed by planar antennas designed for sensing specific wavelengths determined by the geometry of the antennas. A 50-100 ohm heat-sensitive thin-film resistor is commonly used as an antenna load to convert variations of incident optical power into an electrical signal, usually a voltage or current. Although this approach may be promising for some applications, it is generally not fully compatible with existing microbolometer focal plane array technology, as fabricating these antenna-coupled microbolometer detectors involves electron-beam or deep-ultraviolet lithography and a redesign of the underlying readout integrated circuit (ROIC).
Accordingly, various challenges exist in the development of uncooled microbolometer arrays that are operable in the terahertz and far-infrared regions and that could advantageously provide configurable broadband or multi-band absorption spectra.